What role does the president play in his or her political party?

Although the candidates for President are put forward by political Parties, they have no formal role within their Party. They may - because of the prestige of the Presidency - have (even considerable) influence on Party politics and strategy, but there have also been many instances where Presidents' influence on their Party was only very limited. In an ideal world, the President is the person trying to transform his Party's aims into legislation and executive action. In practice, if he faces a hostile Congress with a majority of a rival Party there is only little he can achieve.